It's dative. It's the object or complement of implicitus. Compound verbs usually take a dative complement, which you can think of as an indirect object or the complement of the pre-verb -- in this case in-.

Bear in mind please, that "e" is not an ablative, when the nominative is a neuter and ends on "al, ar, re, e," (animal, exemplar/exemplare/cochlear/altar/altare, mare, rete... and similar) where there would be a danger that accusative, nominative and ablative would all coincide.

In these cases the ablative is always 'ī' (and coincides only with dative). Therefore there is a difference between "in rēte [mittere aliquem]" and "in rēti [esse]"; "in mare [dēsilīre]" and "in marī [esse]"; "in interrēte [aliquid mittere]" and "in interrēti [invenīre aliquid]"; "altāre [accēdere]" and "in altāri [iacēre]"... it will be crucial for you later.

Sorry, I forgot to mention I'm talking about the classical Latin (and its grammar). I would like to be shown decisively in the classical Latin "e" ablative with the neuter nouns of third declension ending on ar,al,e in the nominative.

You can't always know for sure what classical prose authors actually wrote (as opposed to what is transmitted in the mss.), but here's an example that comes with a metrical guarantee (Ovid, Fasti 5.371):

cur tibi pro Libycis clauduntur rete leaenis

The Oxford Latin Dictionary says the ablative singular of rete is usually -e.

Godmy wrote:Bear in mind please, that "e" is not an ablative, when the nominative is a neuter and ends on "al, ar, re, e," (animal, exemplar/exemplare/cochlear/altar/altare, mare, rete... and similar) where there would be a danger that accusative, nominative and ablative would all coincide.

In these cases the ablative is always 'ī' (and coincides only with dative). Therefore there is a difference between "in rēte [mittere aliquem]" and "in rēti [esse]"; "in mare [dēsilīre]" and "in marī [esse]"; "in interrēte [aliquid mittere]" and "in interrēti [invenīre aliquid]"; "altāre [accēdere]" and "in altāri [iacēre]"... it will be crucial for you later.

Godmy I have no clue what you mean by 'the 'e' is not an ablative, when the nominative is a neuter and ends..in...'?

I don't understand this. When the nominative of what is a neuter..and ends ...in...?!

Thank you, Quimmik, for the examples... didn't know about them (or what is said about "rete" specifically in OLD). I was invoking the general rule and I was rather interested whether pmda knew about it or not.